1. Field of the Invention
The invention relates to an in-plane switching type liquid crystal display, and more particularly to an in-plane switching type liquid crystal display which scarcely has display nonuniformity. The invention also relates to a method of operating an in-plane switching type liquid crystal display with less display nonuniformity.
2. Description of the Related Art
A conventional liquid crystal display generally includes a pair of transparent electrodes each formed on a substrate and facing each other, for actuating a liquid crystal layer. In such a liquid crystal display, liquid crystal is actuated in accordance with twisted nematic display process wherein an electric field applied to liquid crystal is directed almost perpendicular to substrates to thereby perpendicularly orient liquid crystal molecules having been horizontally oriented, by the perpendicularly directed electric field.
However, the twisted nematic display process is accompanied with a problem that since the liquid crystal molecules have a certain angle relative to the substrates when perpendicularly oriented by the electric field, brightness varies in accordance with a visual angle, resulting in that it would be impossible to broaden an angle of visibility.
In order to solve the problem, Japanese Patent Publication No. 63-21907, Japanese Unexamined Patent Publication No. 5-505247 (WO91/10936), and Japanese Unexamined Patent Publication No. 7-159786 have suggested an in-plane switching type liquid crystal display including a comb electrode pair, where an electric field to be applied to liquid crystal is directed almost in parallel with substrates to thereby horizontally rotate liquid crystal molecules by the electric field.
Since liquid crystal molecules are kept in parallel with substrates in the above-mentioned in-plane switching type liquid crystal display, it is possible to have a broader angle of visibility than a conventional twisted nematic type liquid crystal display. Hence, an in-plane switching type liquid crystal display is expected to be used as a monitor screen.
FIGS. 1A and 1B illustrate an in-plane switching type liquid crystal display suggested in the above-mentioned Japanese Unexamined Patent Publication No. 7-159786. FIG. 1A is a plan view of the suggested liquid crystal display, and FIG. 1B is a cross-sectional view taken along the line IB--IB.
As illustrated in FIG. 1B, the liquid crystal display is comprised of a first glass substrate 3a, a pixel electrode 4 and a common electrode 5 both formed on the first glass substrate 3a, a second glass substrate 3b spaced away from the first glass substrate 3a by a gap 7 by sandwiching micro-pearls 10 therebetween as a spacer, a liquid crystal layer 8 sandwiched between the first and second glass substrates 3a and 3b, and containing liquid crystal molecules 6, a color filter 9 formed on the second glass substrate 3b so that the color filter 9 faces the first glass substrate 3a, first and second polarizing plates 2a and 2b sandwiching the first and second glass substrates 3a and 3b therebetween, and a back-light source 1 positioned adjacent to the first polarizing plate 2a.
The back-light source 1 is comprised of a light-conductive plate 12, a fluorescent lamp 14 positioned adjacent to an end surface of the light-conductive plate 12, a light scattering sheet 11 formed on an upper surface of the light-conductive plate 12, and a reflection plate 13 formed on a lower surface of the light-conductive plate 12.
In operation, there is generated an electric field between the pixel electrode 4 and the common electrode 5 in accordance with a voltage difference therebetween, to thereby control a rotation angle of axes of the liquid crystal molecules 6 in accordance with an intensity of the electric field.
FIG. 2A illustrates a direction of orientation of liquid crystal molecules when no voltages are applied to the pixel electrode 4 and the common electrode 5, and FIG. 2B illustrates a direction of orientation of liquid crystal molecules when voltages are applied to the pixel electrode 4 and the common electrode 5.
As illustrated in FIG. 2A, when no voltages are applied to the pixel electrode 4 and the common electrode 5, the electrodes 4 and 5 are at the same voltage. Hence, the liquid crystal molecules remain to have initial orientation azimuth. On the other hand, as illustrated in FIG. 2B, when voltages are applied to the pixel electrode 4 and the common electrode 5, the electrodes 4 and 5 are at different voltages, resulting in generation of an electric field therebetween. Hence, the liquid crystal molecules are oriented along the electric field.
In general, the first glass substrate 3a is printed with a sealing material layer for hermetically sealing liquid crystal between the first and second glass substrates 3a and 3b. The sealing material layer also acts as an adhesive for adhering the first and second glass substrates to each other. A plurality of balls called micro-pearl and having a diameter of a couple of micrometers are scattered on a surface of the second glass substrate 3b. When the first and second glass substrates 3a and 3b are adhered to each other, the first and second glass substrates 3a and 3b face each other with a gap equal to a diameter of the micro-pearl therebetween. It is quite difficult to uniformize a gap between the facing glass substrates 3a and 3b, and hence, there would be dispersion in the gap in some areas. Such dispersion in the gap causes unbalance in brightness and chromaticity, which in turn causes display nonuniformity.
Comparing unbalance in brightness and chromaticity caused by dispersion in the gap, unbalance in an in-plane switching type liquid crystal display is greater than unbalance in a conventional twisted nematic liquid crystal display. Hence, an in-plane switching type liquid crystal display is required to have gap uniformity to a greater degree than a conventional twisted nematic liquid crystal display.
FIG. 3 illustrates a relation between chromaticity coordinate values "x" and "y", and a gap formed between substrates in a conventional in-plane switching type liquid crystal display.
For instance, if a gap between substrates is set at 4.5 .mu.m, the substrates are spaced away from each other by 4.5 .mu.m in almost all areas. However, in some areas of the substrates, a gap may be greater or smaller than 4.5 .mu.m, in which case, chromaticity and brightness is varied in accordance with the varied gap. Specifically, when an actual gap is smaller than the initially set gap, displayed images are tinged with blue in chromaticity, and brightness is reduced. When an actual gap is greater than the initially set gap, displayed images are tinged with yellow in chromaticity, and brightness is increased. Thus, there occurs display nonuniformity.
FIG. 4 illustrates a relation between brightness and a gap formed between substrates in a conventional in-plane switching type liquid crystal display. The brightness also varies in accordance with unbalance in the gap, and hence, there occurs display nonuniformity. Actual dispersion in a gap formed between substrates is generally in the range of about .+-.0.5 .mu.m.
As discussed above, chromaticity and brightness vary, if an actual gap between substrates is greater or smaller than an initially set gap. However, it has been found that display nonuniformity is more conspicuous when an actual gap is smaller than an initially set gap than when an actual gap is greater than an initially set gap. This is because that brightness variation caused when an actual gap is smaller than an initially set gap is about twice greater than brightness variation caused when an actual gap is greater than an initially set gap.
As having been mentioned so far, in a conventional in-plane switching type liquid crystal display, if an actual gap between substrates is smaller than an initially set gap in some areas, brightness significantly varies to thereby generate display nonuniformity in the areas.
A liquid crystal display is assembled usually by sandwiching a spherical spacer such as the micro-pearl 10 illustrated in FIG. 1B between two substrates. In assembly of a liquid crystal display, there is generated dispersion in a gap between the substrates for various causes such as a pressure. Such dispersion in the gap causes display nonuniformity. As mentioned earlier, display nonuniformity in an in-plane switching type liquid crystal display is greater than display nonuniformity in a conventional twisted nematic liquid crystal display. Accordingly, an in-plane switching type liquid crystal display provides poor quality in images. One of solutions to the problem of display nonuniformity is to make dispersion in the gap smaller. However, significant improvement in making dispersion in the gap smaller is quite difficult to accomplish in present assembly processes.
Japanese Unexamined Patent Publication No. 63-37785 published on Feb. 18, 1988 suggests a liquid crystal display including a memory for storing uniformity data of each liquid crystal panel, which data is established in accordance with transmissivity measured for each of the liquid crystal panels, and means for calculating compensation data based on both image data of each of the liquid crystal panel and the uniformity data stored in the memory.
Japanese Unexamined Patent Publication No. 63-228127 published on Sep. 22, 1988 suggests a liquid crystal display that includes a pair of glass substrates, a pair of polarizing plates, a filter formed on one of the polarizing plates for cutting long wavelength band, and nematic liquid crystal having an twist angle in the range of 180 to 270 degrees, and chiral material both sandwiched between the glass substrates.
Japanese Unexamined Patent Publication No. 63-296017 published on Dec. 2, 1988 suggests a liquid crystal display having a liquid crystal layer containing chiral material, and nematic liquid crystal having an twist angle in the range of 180 to 270 degrees, the nematic liquid crystal further containing material which has high absorbance in long and short wavelength bands.
Japanese Unexamined Patent Publication No. 5-249444 published on Sep. 28, 1993 suggests a liquid crystal display including a liquid crystal layer having a central portion thicker than marginal portions thereof